In previous posts on 24 May 2015 and 28 June 2016, I reported on the TOP500 rankings of the world’s supercomputers.

In June 2013, China’s Tianhe-2 supercomputer at the National Supercomputer Center in Guangzho topped this this worldwide ranking with an Rmax Linpack score of 33 petaflops (1 petaflops = 1015 floating-point operations per second) and retained the first place position for two years. In June 2016, the new leader was another Chinese supercomputer, the Sunway TaihuLight at the National Supercomputer Center in Wuxi. TaihuLight delivered an Rmax Linpack score of 93 petaflops and remained at the top of the worldwide ranking for two years, until it was eclipsed in June 2018 by the US Summit supercomputer, then with an Rmax rating of 122.3 petaflops.

In the latest TOP500 ranking, the new leaders are two US supercomputers: Summit (#1) and Sierra (#2).

Summit supercomputer. Source: NVIDIA

The IBM Summit improved its past Linpack score to achieve an Rmax of 143.5 petaflops in the current ranking. Summit is located at the Department of Energy (DOE) Oak Ridge National Laboratory (ORNL) in Tennessee.

The IBM Sierra also improved its past Linpack score to achieve an Rmax of 94.64 petaflops / second and move into second place, marginally ahead of China’s TaihuLight. Sierra is located at the DOE Lawrence Livermore National Laboratory (LLNL) in California.

The TOP500 ranking places LANL’s Trinity supercomputer (a Cray XC40) as the #6 fastest supercomputer in the world, but its performance (Rmax of 20.16 petaflops) is far below that of the #1 Summit supercomputer at Oak Ridge national Laboratory and the #2 Sierra supercomputer at Lawrence Livermore National Laboratory.

Source: LANL

Not to be outdone, LANL issued a request for proposal (RFP) in February 2019 for a new supercomputer, to be named Crossroads, to support the lab’s missions for the National Nuclear Security Administration (NNSA). A LANL spokesperson reported that, “High performance computing across the NNSA complex is used to assure the safety, security and effectiveness of the U.S. nuclear deterrent; to analyze and predict the performance, safety, and reliability of nuclear weapons and certify their functionality.” Responses to the RFP are due by 18 March 2019. Crossroads is expected to go online in 2021.

Radioisotope Thermoelectric Generators (RTG), also called Radioisotope Power Systems (RTS), commonly use non-weapons grade Plutonium 238 (Pu-238) to generate electric power and heat for National Aeronautics and Space Administration (NASA) spacecraft when solar energy and batteries are not adequate for the intended mission.

Approximately 300 kg (661 lb) of Pu-238 was produced by the Department of Energy (DOE) at the Savannah River Site between 1959 – 1988. After U.S production stopped, the U.S. purchased Pu-238 from Russia until that source of supply ended in 2010.

Limited production of new Pu-238 in the U.S re-started in 2013 using the process shown below. This effort is partially funded by NASA. Eventually, production capacity will be about 1.5 kg (3.3 lb) Pu-238 per year. The roles of the DOE national laboratories involved in this production process are as follows:

Idaho National Engineering Lab (INEL):

Store the Neptunium dioxide (NpO2) feed stock

Deliver feed stock as needed to ORNL

Irradiate targets provided by ORNL in the Advanced Test Reactor (ATR)

Return irradiated targets to ORNL for processing

Oak Ridge National Lab (ORNL):

Manufacture targets

Ship some targets to INEL for irradiation

Irradiate the remaining targets in the High Flux Isotope Reactor (HFIR)

The U.S. has an existing inventory of about 35 kg (77 lb) of Pu-238 of various ages. About half is young enough to meet the power specifications of planned NASA spacecraft. The remaining stock is more than 20 years old, has decayed significantly since it was produced, and does not meet specifications. The existing inventory will be blended with newly produced Pu-238 to extend the usable inventory. To get the energy density needed for space missions while extending the supply of Pu-238, DOE and NASA plan to blend “old” Pu-238 with newly produced Pu-238 in 2:1 proportions.

NASA slowly has been developing an Advanced Stirling Radioisotope Generator (ASRG), which should be capable of producing about four times the power of older RTGs per unit of Pu-238. However, the ASRG produces less waste heat, which can be used productively to warm electronics in the interior of a spacecraft, such as the Mars rover Curiosity. The ASRG may not be available in time for the next space mission requiring an RTG power source, in which case an existing RTG design will be used.

Read a history of RTGs and more information on current U.S. Pu-238 production status in a 2014 presentation by Ralph L McNutt, Jr, at the following link:

On 22 December 2015, DOE reported production of 50 grams of new Pu-238.

DOE reported that it plans to set an initial production target of 300 – 400 grams (about 12 ounces) of Pu-238 per year. After implementing greater automation and scaling up the process, ORNL expects to reach the the production target of 1.5 kg (3.3 lb) Pu-238 per year.

The next NASA mission that will use an RTG is the Mars 2020 rover, which will use the same Multi-Mission Radioisotope Thermoelectric Generator (MMRTG) as used on NASA’s Mars rover Curiosity. MMRTG can provide about 110 watts of electrical power to a spacecraft and its science instruments at the beginning of a mission.

You can read the ORNL announcement of initial Pu-238 production at the following link:

In the past three years, ORNL has made scant progress in producing Pu-238. In a 13 December 2018 article, “NASA Doesn’t Have Enough Nuclear Fuel For Its Deep Space Missions,”author Ethan Siegel reports: “Although current production (at ORNL) yields only a few hundred grams per year (less than a pound), the laboratory has the eventual goal of ramping up to 1.5 kilograms (3.3 pounds) per year by 2023, at the earliest. Ontario Power Generation in Canada has also begun producing Pu-238, with the goal of using it as a supplemental source for NASA.” You can read the complete article on the Forbes website at the following link:

The Canadian plans for becoming a source of Pu-238 was announced on 1 March 2017: “Ontario Power Generation (OPG) and its venture arm, Canadian Nuclear Partners, are participating in a project to produce isotopes in support of deep space exploration. Under the agreement, OPG would help create isotopes at the Darlington nuclear station east of Toronto that will help power space probes.” You can read the complete OPG press release here:

More details are in Los Alamos report LA-UR-12-20494, “Our Next Two Steps for Fukushima Daiichi Muon Tomography”, which is the source of the above diagram. You can view or download this LANL report for free at the following link: